Multiple Populations and a CH Star Found in the 300S Globular Cluster Stellar Stream

TL;DR

This study analyzes eight red-giant members of the 300S stellar stream with high-resolution spectroscopy to test MSP formation in a globular-cluster progenitor. It identifies a second-population star, constrains the stream’s metallicity dispersion to $<0.09$ dex, and estimates the progenitor’s initial mass to be $M_{ m Ini} \sim 10^{4.5-4.9}\,M_{igodot}$, placing 300S near a possible MSP-threshold. The detection of a CH star indicates binary-mass-transfer signatures, while the Mg/Fe and orbital properties support an ex situ origin linked to Gaia-Sausage-Enceladus rather than a Milky Way dwarf galaxy. Overall, the work highlights how globular cluster MSPs, initial mass thresholds, and binary survival can be probed through disrupted GC streams.

Abstract

Milky Way globular clusters (GCs) display chemical enrichment in a phenomenon called multiple stellar populations (MSPs). While the enrichment mechanism is not fully understood, there is a correlation between a cluster's mass and the fraction of enriched stars found therein. However, present-day GC masses are often smaller than their masses at the time of formation due to dynamical mass loss. In this work, we explore the relationship between mass and MSPs using the stellar stream 300S. We present the chemical abundances of eight red giant branch member stars in 300S with high-resolution spectroscopy from Magellan/MIKE. We identify one enriched star characteristic of MSPs and no detectable metallicity dispersion, confirming that the progenitor of 300S was a globular cluster. The fraction of enriched stars (12.5\%) observed in our 300S stars is less than the 50\% of stars found enriched in Milky Way GCs of comparable present-day mass ($\sim10^{4.5}$\msun). We calculate the mass of 300S's progenitor and compare it to the initial masses of intact GCs, finding that 300S aligns well with the trend between the system mass at formation and enrichment. 300S's progenitor may straddle the critical mass threshold for the formation of MSPs and can therefore serve as a benchmark for the stellar enrichment process. Additionally, we identify a CH star, with high abundances of \textit{s}-process elements, probably accreted from a binary companion. The rarity of such binaries in intact GCs may imply stellar streams permit the survival of binaries that would otherwise be disrupted.

Figures (8)

• Figure 1: The positions (top panel) and S$^5$ radial velocities (bottom panel) of stars in the region of 300S. Green stars represent 300S stars analyzed in this work. Red crosses represent stars rejected from this work as radial velocity non-members. The four red crosses to the right (at 153 degrees right ascension) represent the multiple radial velocities measured for the most metal-poor star analyzed by Fu2018. Blue points represent 300S member stars identified in DECam photometry and S$^5$ spectroscopy. Gray dots represent other stars observed by S$^5$.
• Figure 2: Isochrones matched to DECam DECaLS DR9 photometry. The blue and orange lines represent the best-fit isochrones found in this article and using parameters from Fu2018 and Li2022. The parameters used for the latter isochrone are: age = 12.5 Gyr, [$\alpha$/Fe] = 0.4, [Fe/H] $= -1.27$, d = 17.2 kpc, where [Fe/H] and d were from Li2022 and the age and [$\alpha$/Fe] abundance are obtained from Fu2018. Our best-fitting isochrone parameters are: age = 12.5 Gyr, [$\alpha$/Fe] = 0.0, [Fe/H] $= -1.35$, d = 17.7 kpc. The green stars, and blue circles represent the dereddened color and magnitude of stars analyzed in this work and other member candidates observed by S$^5$. The DECam data is a background-subtracted Hess diagram along 300S' sky position.
• Figure 3: A comparison of normalized spectra around an O line, two Na lines, and the C-N bands in the top, middle and lower panels, respectively. We highlight the Telluric lines in the top panel. We emphasize here the enriched star J1020$+$1555 (dashed green line) and the star with most similar stellar parameters, J0956$+$1603 (red solid line). The former has a higher Na and N enrichment and O depletion as expected of enriched stars in GCs. The spectra of the other stars, which are significantly colder than the highlighted stars, are shown as thin gray lines. The bright red dotted line indicates the wavelength of the measured absorption lines.
• Figure 4: The abundances of elements characteristic of multiple population enrichment: nitrogen, oxygen, and sodium in the left, middle and right plots, respectively. The purple circles represent the abundances of the ordinary stars. The anomalous stars J1020$+$1555, J1001$+$1554, and J1045$+$1408 are marked with a dark red star, a green cross, and a blue triangle respectively. J1020$+$1555 has the requisite enrichment in N and Na, and depletion in O. We also measure unusual of Na enrichment in the cool star, J0956$+$1603. Additionally, in the rightmost plot, the black points represent the NLTE-corrected abundances. However, due to the correlation between $T_\mathrm{eff}$ and the Na abundance and no depletion in O, we attribute this to an unknown systematic error and do not consider this a second-population star.
• Figure 5: Correlated abundances of element Na compared to N and O in the left and right plots, respectively. The purple circles represent the abundances of the ordinary stars. The anomalous stars J1020$+$1555, J1001$+$1554, and J1045$+$1408 are marked with a dark red star, a green cross, and a blue triangle respectively. The black points represent the NLTE-corrected abundances. J1020$+$1555 has the requisite enrichment in N and Na, and depletion in O. We also measure unusual of Na in the cool star, J0956$+$1603. However, due to the correlation between $T_\mathrm{eff}$ (as seen in Fig. \ref{['fig:second_pop']}) and the Na abundance and no depletion in O, we attribute this to an unknown systematic error and do not consider this a second-population star.